Protective device



oct. 27, 1942. A, J. Mmmm f 2,300,127

` I RoTEcTIvE' DEVICE Filed oct. 11. 1940 F ig. 2.

lIl l fIToRNeY Patented Oct.' 27, 1942 PROTECTIVE DEVICE Alan JulianMaddock, London, England, assigner to lVestern Electric Company,Incorporated, New York, N. Y., a corporation of New York ApplicationOctober 11, 1940, Serial No. 360,764 In Great Britain October 20, 1939(Cl. Z50-'17) Claims.

It is the purpose of this invention to provide protection for apparatusinvolving the use of electron discharge valves employed as ampliers ofelectric waves or as generators thereof. A particular object of theinvention is the provision of protection for radio transmitters againstdamage by lightning or breakdown of the apparatus and for low frequencyamplifiers against overload. i

It is a further object of this invention to provide such protection withthe minimum delay of action and a circuit is employed which obviates thenecessity for any relays or contactors.

In a radio transmitter, for example, supplying radio frequency power toan antenna system, serious damage may be caused to the equipment if alightning discharge occurs to the antenna. The lightning may initiate anarc or flashover between some part of the electrical circuit and earthand through the arc so initiated the transmitting equipment will thensupply considerable radio frequency power and it is this power whichoften causes the greatest damage to the apparatus. Means must thereforebe provided for removing the anode voltage supply to some part of thetransmitter or to render at least a part of the amplifying chaininoperative as an amplier in order to extinguish this power arc. Onceextinguished, normal operation may then be resumed since the lightningdischarge has ceased and so the arc will not be initiated again.

According to the invention, the arc initiated by the lightning dischargeor iiashover acts as a form of electronic switch to connect into thecircuit of one or more of the valves a source of voltage which rendersinoperative the normal amplifying or oscillating action of the valve crvalves. In one arrangement for carrying out the invention a negativebias is applied to a grid of one or more valves in the amplifying chain,preferably one of the earlier stages in the amplifier. By this means thesaid valve or valves and the succeeding valves are rendered inoperativeas ampliers so that no power lis generated to feed into the arc. The arcthus ceases, the

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bias thereby being removed and the amplifiers are then able to operateagain normally.1

The invention will be better understood from a consideration of thefollowing descriptiony in conjunction with the accompanyingdrawing,Figs. l, 2 and 3 of which show three methods of applying the inventionto a radio transmitter. Referring to Fig. 1 V is a thermionic valveacting as an amplifier in the radio frequency amplifying chain and isshown for convenience as a triode but it will be clear from thefollowing description that the system may be applied equally well to atetrode, pentode or other multielectrode tube, said tube acting eitheras ampliiier or as oscillator.

T represents the point at which protection against a discharge isrequired and may be the antenna system connected to the ampliers or thetransmission line or concentric feeder connecting said amplifier to theantenna system: in any case G represents a point Vwhere a discharge toearth may occur and a definite point may be arranged for G if desiredsuch as horn or ball gaps though it is clear that the system willfunction no matter where the arc occurs so long as there is a connectionfor direct current to the point of iiashover.

The grid of V is supplied with its normal D. C. potential by the sourceB1 through a resistance R whose function will be described later. In thediagram the supply is shown for convenience as being connected to thegrid through a radio frequency choke L3 but obviously any known schememay be employed. The source B1 and cathode of V are connected to earth.

From the grid side of resistance R is connected a source of supply B2with itsv negative pole connected to R and its positive pole connectedto the antenna system, transmission line or other point where protectionis required (designated by T) through a radio frequency filter systemsuch, for example, as shown consisting of chckes and condensers L1, L2and C, the purpose of this being to provide a D. C. connection to Twhilst preventing radio frequency voltage being communicated to B2 andthe remainder of the system.

The method of operation may now be described as follows:

Under normal conditions valve V has impressed upon its grid the normalD. C. voltage supplied by the source B1 but when an arc occurs between'I' and earth, i. e., across the gap G, the arc acts as an electronicswitch and connects the positive pole of B2 to earth so that the voltagenow existing between the grid and cathode of V is negative by thedifference between the supply voltage of B2 and the arc drop across G.If the value of B2 is correctly chosen the grid of V will be made suchan amount negative, or substantially Zero if preferred, with respect tothe cathode that V ceases to operate as an amplifier and the whole ofthe succeeding stages receive no radio frequency driving voltage and sono power Vis delivered to the point T. The main power arc across G thusceases and the only current that might tend to persist across G is thatdetermined by sources B1 and B2 acting in series (opposing in the caseillustrated) and flowing through resistance R. The value cf R andvoltage of B2 are so chosen that the current from B1 and Bz through thearc is of such value that the arc is not maintained- As soon as the arcceases B2 is automatically disconnected from earth and only the normalgrid voltage from B1 is applied to the grid of V and the amplifier thenagain functions normally. The protective circuit is then ready tofunction when any further discharge occurs across G.

It will be seen from the foregoing that the system operates by theapplication of a zero or negative potential to the grid of a thermionicvalve during the time of passage of an arc between one point of thesystem and earth. It is clear, therefore, that the system is not limitedto the use of a triode valve as described above but may be applied to atetrode, pentode or other multielectrode tube and may be applied tocontrol of voltage not only of the control grid but of the screen grid,suppressor grid, or any other electrode where the application of asubstantially zero or negative voltage renders the valve inoperative inits normal manner including an electrode specifically added to the valvefor use in the manner described.

It is clear also that the scheme is not limited to use in radiofrequency amplifiers, which latter may be of any type and amplifyingmodulated waves or not, but may be used equally well for so calledaudio-frequency or vision-frequency amplifiers or any amplifiers oroscillators where thermionic valves are employed as amplifiers oroscillators to provide protection against damage by flashover oroverload. In this case the protective filter would be made suitable forthe frequencies involved. An extension of the scheme is also possiblewhere it is desired to suppress both radio and audio-frequencyamplification at the same time, for example in transmitters employingfinal stage anode modulation where considerable audio-frequency power isdealt with and both amplifying chains may be rendered inoperative.

The scheme may also be extended to cases where it is desired to secureprotection against discharges that may occur over any of a number ofpoints, for example, in an antenna array consisting of several antennaefrom which discharges to earth may occur, said antennae being fed by onetransmitter. In this case separate R. F. filters such as L1, Lz, C, maybe connected to each antenna on the one side and on the other allconnected to the source Bz as shown in Fig. 2: in this way a dischargeto earth on any of the antennae will cause the transmitter to ceasefunctioning momentarily.

A still further application is to the case where two or moretransmitters are connected to the same antenna system in which case aseparate suppressor source of voltage B2 may be used for eachtransmitter but the positive pole of all these supplies may be connectedto the same R. F. filter system L1, L2, C. By this means alltransmitters are rendered inoperative when a discharge occurs from theantenna system to earth. Such a scheme is shown diagrammatically in Fig.3.

If desired the rate at which restoration of the amplifying chain returnsto normal can be adjusted to a certain extent by suitable choice of thevalue of resistance R and by connecting a condenser across thisresistance.

What is claimed is:

1. In amplifying apparatus to be protected against discharge damage bylightning or overload, a thermionic valve having a grid, a source ofvoltage and a connection from said grid over said Source of voltage to apoint in said apparatus where protection is required, whereby theinitial arc or flashover acts as a form of electronic switch to connectsaid source of voltage into the circuit of said valve to renderinoperative the normal amplifying action of said valve.

2. In amplifying apparatus to be protected against discharge damage bylightning or overload, a thermionic valve having a grid and a cathode,said cathode being grounded, a source of voltage and a connection fromsaid grid over said source of voltage to a point in said apparatus whereprotection is required, whereby an arc or flashover to ground acts as aform of electronic switch to connect said source of voltage into thegrid-cathode circuit of said valve to render inoperative the normalamplifying action of said valve.

3. In amplifying apparatus to be protected against discharge damage bylightning or overload, a thermionic valve comprised in said apparatus, agrid and a cathode in said valve, a source of voltage, a connection fromsaid grid to the negative side of said source of voltage, and aconnection from the positive side of said source of voltage to a pointin said apparatus where protection is required, whereby the initial arcor fiashover acts as a form of electronic switch to connect said sourceof voltage into the circuit of said valve to render said grid ofnegative potential with respect to said cathode and make inoperative thenormal amplifying action of said valve.

4. In amplifying apparatus including a thermionic valve having a gridand a cathode, a source of voltage for rendering said grid sufficientlynegative to make inoperative the normal amplifying action of said valve,said grid being connected over said source of voltage to a point in saidamplifying apparatus where protection from discharge damage is required,a normal source of grid voltage for said tube and a resistance connectedin series with said normal source of grid Voltage.

5. The combination in accordance with claim 4 in which said resistanceand said normal source of grid voltage are serially connected betweensaid grid and cathode, and said first-named source of voltage has itsnegative side connected to the grid end of the resistance and itspositive side connected to the point where protection is required.

6. The combination in accordance with claim 4 in which said resistanceand said normal source of grid Voltage are serially connected betweensaid grid and cathode, said cathode being ground* ed and saidfirst-named source of voltage has its. negative side connected to thegrid end of the resistance and its positive side connected to a pointwhere protection is required against an arc. or flashover to ground.

7. Amplifying apparatus including a thermicnic Valve having a grid, asource of voltage and a protective filter, said grid being connectedover said source of voltage and said protective filter to a point insaid amplifying apparatus Where protection against discharge damage isrequired.

8. Amplifying apparatus including a thermionic, valve having a grid, asource of voltage, a pro-4 tective lter and an antenna, said grid beingconnected over said source of voltage and said protective lter to apoint of said antenna where protection from damage due to arc orflashover is required.

9. A radio transmitter including a thermionic valve having a grid, asource of Voltage, a plurality of protective filters and a plurality ofantennae, said grid being connected over said source of voltage andthrough a plurality of said protective lters respectively to saidantennae.

10. A plurality of radio transmitters, each having a thermionic valvewith a grid, an antenna system, a plurality of sources of voltage, and aprotective filter, connections from each of said radio transmitters tosaid antenna system, a. connection from each of said grids to thenegative side of a separate one of said sources of voltage,v

